1. Field of the Invention
This invention relates to a system for compensating for cross polarization coupling which is needed for the reuse of a frequency wave by orthogonal polarization in a satellite communication system.
2. Description of the Prior Art
Generally, the reuse of a frequency wave by the employment of two polarized waves having the same frequency but polarized orthogonal to each other will enable the traffic capacity to be increased substantially twice or so. In practice, however, polarization distortion by the rainfall and incomplete polarization characteristics of an earth station antenna and a satellite antenna offer a serious obstacle to the realization of a dual-polarization satellite communication system. Especially, the orthogonality is seriously degraded by the rainfall and the amount of its influence differs with time, regions, frequency bands used, etc. Consequently, an important key for the realization of the dual-polarization satellite communication system is how to compensate for cross polarization coupling caused by the rainfall.
By the way, a main cause of cross polarization coupling by the rainfall in frequency bands (for example, 6 GHz and 4 GHz bands) assigned to satellite communications is as follows: That is, a raindrop is not completely spherical but elliptic, so that the propagation delay for an electric wave is maximum or minimum in the major axis M or the minor axis L of the ellipse, respectively. This results in the polarization dependency of the delay. In this case, an angle between the direction of the equivalent major axis M of the raindrop and the horizon is called an effective canting angle (hereinafter referred to as ECA), while a difference between the maximum delay and the minimum delay is called a differential phase shift (hereinafter referred to as DPS).
For compensating for cross polarization coupling, it is necessary to detect ECA and DPS in both of an up-link (a path from the earth to a satellite) and a down-link (a path from the satellite to the earth) during rainfall. It is known in the art that if ECA and DPS can be detected, compensation for cross polarization coupling in the both links can be achieved by inserting in a transmitter or a receiver a dielectric plate having a delay equal to a detected DPS.
It is also known that the compensation for cross polarization coupling in the down link can be effected if the earth station receives a beacon wave from a satellite and detects the level and phase of the received wave for DPS and ECA, respectively.
For compensating for cross polarization polarization in the uplink, it is necessary for the station effecting the compensation to send a pilot signal and receive the same through the satellite. Heretofore, there has been proposed a method of sending two pilot waves of different frequencies in an orthogonal relationship to each other for the detection of ECA and DPS. By this method, the detection of DPS can be achieved by comparing the received levels of the two pilot waves sent back from the satellite with each other but the two pilot waves are needed therefor, and since information of a phase difference between the two pilot waves cannot be utilized because of a frequency difference there between, ECA in the up-link cannot be directly detected.